You will find Heir in the lower courtyard of Skyhold, between the stables and the kitchen. Speak with him to begin the quest ‘Way of the Assassin’. To complete the quest and unlock the Way of the Assassin skill tree, you will need to craft a unique Murder Knife. This can be done at a requisition table once you have the following items:
Codex Entry: Writing on Assassin’s methods - Found on a table near Cole in Skyhold’s tavern, the Herald’s Rest. Can also be purchased from the merchant Willvan in Val Royeaux
3x Assassin’s Guild Leader Tokens - Obtained in the Crestwood regions of East Side Hills, The Black Fens, and The Flats by killing Assassin leaders
20x Deathroot - This item can be purchased from the merchant in Crestwood village. It can also be looted throughout the regions of Western Approach and Hissing Wastes, or from the war table mission Hissing Wastes Resources by selecting Leliana
Note: You will need to complete the war table mission Specializations for the Inquisitor before you can find and speak with the trainer.